"Trukk not Munky" (or "Truk not Munky") is a phrase used by Transformers fans derisively towards those that complain about changes to the mythos.

How did this phrase come into use? The answer is surprisingly hard to find; history is quickly lost online. Some records do exist, however, such as the alt.toys.transformers archives.

During Beast Wars, very serious posts bemoaning that "Optimus should be a truck, not a stupid monkey" were not uncommon, as indicated by the Usenet quote above. They took various forms, and were quickly parodied. Rob Powers was particularly fond of these parodies, which he usually formed as variations on the phrase "Why is Optimus a stupid monkey instead of a cool truck", with assorted intentional misspellings and cacography for humorous effect. By June of 2000 [original post], his parody seems to have become fairly standardized in form, but it was not "TRUKK NOT MUNKY" yet.

This condensed form first appeared in early 2001 (the single-K form seems a few months older, but the doubled K soon eclipsed it). The new phrase was a further condensing of the earlier forms, apparently conceived by Recharge in an IRC-related search for a band name. [original post] Being brief, easily understood by most fans, and amusing, "TRUKK NOT MUNKY" quickly became the go-to sarcastic reply.
Since the Beast Era has ended, the term's usage has been generalized to indicate knee-jerk reactionary complaints of a "it's not what we had before (even if we did) so it's bad" nature. Such as the lesser used "Jett not Monky" variant regarding Starscream's new simian-like look in the movie franchise.

Of course, the whole point is rendered moot when you realize that it is Optimus Primal, not Optimus Prime, transforming into a "Munky," and that Optimus Primal's alt mode is actually an ape, specifically a gorilla, not a monkey.[1] (This has been understood by the humorous users of the term all along, and adds approximately 34.5% of the snide.)

Notes

The "Trukk not Munky" meme was later used by Masters of the Universe Classics fans to describe Gygor, a giant yellow and green gorilla released in the new line.

One of the Botcon videos have a "Trukk not munky." Reference also when Optimus Prime says "Optimus Prime is a truck, not a poop slinging monkey."[Detail needed]

Japan picked up on the joke when Gorisaki Banana, who is voiced by Tessho Genda had a vehicle form as a Truck.